The present invention relates to a novel chitin-binding receptor kinase and the gene thereof encoding the receptor kinase isolated from tobacco.
Particulary, the present invention relates to a receptor kinase having a specific binding activity to oligomers or polymers of chitin molecules and its gene derived from Nicotiana tabacum. 
The said receptor kinase has an extracellular domain homologous to other chitinases which are chitin-degrading enzymes, and its gene expression is specifically induced by infection of tobacco mosaic virus(TMV) and fungal pathogen. The receptor kinase and its gene in the present invention is regarded as a chitin signal transduction receptor localized in the cell membrane which activates plant defense systems, and thus makes plants operate the defense systems to various kinds of pathogens.
Following the invasion of a plant by a potential pathogen, the pathogen either successfully proliferates in the host, causing associated disease symptoms, or its growth is halted by the defenses of the host plant. One such defense is the hypersensitive response (HR), a rapid cellular necrosis near the site of the infection that correlates with generation of activated oxygen species, production of antimicrobial compounds and reinforcement of host cell walls. Other defenses include systemic acquired resistance (SAR), which effectively protects the plant against subsequent attack by a broad range of pathogens.
A number of plant disease resistance genes activated by signal molecules of pathogens have been cloned, and similar features have been discovered among many of these resistance genes, in spite of the diversity of pathogens against which they act. These features include a leucine-rich-repeat (hereinafter, referred to as xe2x80x98LRRxe2x80x99), a motif found in a multitude of eukaryotic proteins with roles in signal transduction(Kobe and Deisenhofer, Trends Biochem. Sci., 9, 415-421, 1994). The LRR motif is thought to be involved in protein-protein interactions and may allow interaction with other proteins that are involved in plant disease resistance. In addition, nucleotide binding sites and leucine zippers are shared among many resistance genes(Dangl, Cell, 80, 383-386, 1995; Staskawicz et al., Science, 268, 661-667, 1995). These motifs are presented and similarly organized among resistance gene products from plants as diverse as tobacco, tomato, rice, flax, and Arabidopsis, suggesting a common mechanism underlying disease resistance signal transduction throughout the plant kingdom.
Recently, it is known that the plant receptor kinase is a factor of signal transduction of plant defense. It is revealed that many kinds of receptor kinases in plants are involved in hormone reactions, organ developments, and interactions between plants and pathogens. Several kinds of R(resistance) gene of plants which show resistance to specific types of pathogens have been identified as receptor kinase genes. (Wang, X. et al., Proc. Natl. Acad. Sci., 93, 2598-2602, 1998).
The plant receptor kinase is classified into several groups based on the amino acid sequence of the extracellular domain. However, a receptor kinase which contains a chitinase-like domain has not been reported.
It is well known that chitinase activity increases dramatically after pathogen invasion and this is presumably due to the host plant""s attempts to degrade the chitin of the fungal cell wall. Chitin is an important elicitor activating the plant defense system to increase the level of reactive oxygen species(ROS), synthesize phytoalexins, and stimulate transcriptions of several genes of defense proteins. Hitherto, the general transduction mechanism of chitin has not been reported, but several evidences that the signal transduction is initiated by recognition between receptor and the elicitor have been reported. In addition, chitinase has been shown in vitro to inhibit fungal and insect growth, and in transgenic plants a bacterial chitinase has been shown to exhibit inhibitory effects towards numerous pathogens and pests including insects.
Until now, many researchers have attempted to develop plants having a resistance to fungi by activating defense systems that plants originally own. Actually, overexpression of chitinases or glucanases in infected plants as defense proteins have been observed. At the gene level, it is revealed that a species-specific resistance gene from Zea maize (corn), Hm1 confers resistance against specific races of the fungal pathogen Cochliobolus carbonum by controlling degradation of a fungal toxin(Johal and Briggs, Science, 258:985-987, 1992).
However, development of resistant plants using the defense systems has failed, and even a successful example has not reached the practical stage because of its petty effects. It shows that it is impossible to obtain enough resistance for practical use by expression of one or two plant defense proteins.
Therefore, in order to obtain plants having a high resistance to pathogens, it must be considered to use whole plant defense systems corresponding to infection of pathogens. The most prominent strategy for obtaining resistant plants is to make plants recognize rapidly the infectious signal of pathogens and activate more effective defense systems. For the purpose of obtaining plants highly resistant to many kinds of fungi, isolation of the receptor kinase genes involved in signal transduction related to plant defense systems has been required.
In order to develop pathogen-resistant plants with activated defense systems, these inventors have investigated and purified a novel receptor kinase containing the extracellular domain similar to chitinase derived from Nicotiana tabacum. 
Thus, these inventors have demonstrated that this novel receptor kinase may represent a receptor protein of plant defense systems regulating chitin signal transduction, of which chitinase domain binds to chitins in an extracellular space and activates the kinase domain in a cytoplasm.
It is an object of this invention to provide a novel receptor kinase CHRK1 and the gene thereof for developing pathogen-resistant plants.
In accordance with the present invention, the foregoing objects and advantages are readily obtained.
The present invention provides the novel receptor kinase which contains chitinase related sequence and has a binding activity to chitins.
Particularly, this invention provides a receptor kinase CHRK1 derived from Nicotiana tabacum described by amino acid sequence of SEQ ID NO: 1.
In addition, this invention provides the cDNA sequence of receptor kinase described by nucleotide sequence of SEQ ID NO: 2 derived from Nicotiana tabacum. The receptor kinase gene in the present invention can be detected in Zea maize, Oryza sativa, Perunia inflata, or Brassica oleracea. 
This invention also provides the plasmid vector pCHRK1 comprising the receptor kinase cDNA (Accession NO: KCTC 0561BP).
Further objects and advantages of the present invention will appear hereinafter.